1. Field of the Invention
The present invention relates to a process for producing a copolymer which has low yellowness and high transparency and which is useful for transparent parts such as optical parts, automobile parts, and electric and electronic parts.
2. Description of the Related Art
In recent years, application of transparent plastics is actively investigated for optical parts such as optical lenses, and optical fibers; automobile parts such as headlight lenses and sunroofs, and the like for productivity and light weight of the parts. With progress of the investigation, the requirement for the properties of the transparent plastics are becoming severer increasingly. In particular, improvement of the heat resistance thereof is strongly demanded.
Copolymers having succinimide units have been investigated comprehensively because of their high heat resistance. For example, copolymerization of methyl methacrylate with N-aromatic-substituted maleimide is disclosed in JP-B-43-9753, JP-A-61-141715, JP-A-61-171708, and JP-A-62-109811; incorporation of N-aromatic-substituted maleimide into a styrene type resin by copolymerization is disclosed in JP-B-47-6891, JP-A-61-76512, and JP-A-61-276807. At the higher content of the N-aromatic-substituted maleimide, the resins obtained by these methods have higher heat resistance, but involve disadvantages of brittleness, poor processability, discoloration, and so forth.
On the contrary, copolymers constituted of succinimide unites and olefin units are superior in heat resistance and mechanical properties.
British Patent 815821 discloses a process for producing a succinimide copolymer by reaction of a maleic anhydride-olefin copolymer with a methylamine. In this process, a maleic anhydride copolymer is amidated with methylamine in benzene, then the solvent is removed, and the residue is heated in an oven to convert the formed amide to an imide to obtain a pale yellow resin. The color tone of the polymer is shown to be improved by eliminating the remaining maleic anhydride and conducting the reaction in an inert atmosphere.
Journal of Polymer Science, Part C., No.16, p.387 (1967) describes an imide-forming reaction (imidation) of maleic anhydride copolymer with an alkylamine in acetic acid or benzene.
The copolymers obtained by the above methods, however, are not sufficient yet to meet the demand for the transparent plastics although the yellowness is somewhat mitigated.
After comprehensive studies to solve the above problems, it was found by the inventors of the present invention that the above problems are solved by a succinimide unit-containing maleic anhydride-olefin copolymer produced by reaction of a maleic anhydride-olefin copolymer with an amine and subsequent ring-closing imidation in a slurry state in a poor solvent, and the present invention has been completed.